There are many known aqueous emulsion polymers that are used in a variety of applications, including adhesive applications. Vinyl acetate-ethylene (VAE) emulsion polymers have been preferred in water-based packaging adhesive applications primarily because of the low cost of production, availability of reactants, the adhesion, setting speed, and wet tack properties they offer on glass, metal, corrugated and other paperboard materials, and the ease at which they can be applied and maintained on adhesive application equipment.
Over the past 8-10 years there has been a continuous shift in the type of packaging materials utilized in the industry. Today the segment which is based on plastic or polymeric materials is growing and rapidly replacing the traditional materials mentioned above. This new class of plastic packaging is far more difficult to adhere to than glass, metal and paper-based packaging materials which have been in use.
Various methods have been used to improve the bonding of VAE adhesives to difficult to bond substrates, such as polyethylene, polypropylene, poly(ethylene terephthalate), and surfaces having low surface energy, low polarity, or little or no porosity. Surface treatments, such as plasma treatment, corona discharge, or chemical oxidation, have been used to alter the surface so that they can more easily be bonded with VAE adhesives; however these treatments are expensive and time consuming, and are not always possible.
In order to achieve adhesion on difficult to bond surfaces, traditional VAE adhesive formulations have been highly plasticized or formulated. However, introduction of additional compounding aids adds to the complexity and cost of the formulations and, frequently leads to problems during the application and/or machining of the resulting adhesives. In addition, the list of additives available to the adhesive formulators is decreasing because many of the solvents and plasticizers used in the past are no longer environmentally acceptable.
Another approach to improve the bonding of VAE adhesives to low polarity surfaces has been to add other monomers to the polymer. For example, U.S. Pat. No. 5,371,137 (Blincow et al. 1994) discloses VAE copolymer emulsions to which about 5% to about 85% of vinyl esters of C.sub.4 to C.sub.18 primary or secondary carboxylic acids have been added as a monomer. U.S. Pat. No. 5,500,251 (Burgoyne et al., 1996) disclose the incorporation of the following compounds into VAE systems to promote adhesion to low energy polyolefin surfaces: N-(4-alkylphenyl)acrylamides, N-(4-alkylphenyl)methacrylamides and N-(4-alkylphenyl)maleimides.
VAE pressure sensitive adhesives will adhere to most surfaces but lack of both cohesive strength and wet tack limits the use of these adhesives on difficult to bond surfaces, particularly when the surfaces are exposed to high temperatures. U.S. Pat. No. 4,128,518 (Oyamada et al.) discloses a VAE pressure sensitive adhesive containing a base material of an aqueous emulsion of vinyl acetate-ethylene copolymer having an ethylene content of 15 to 40% by weight, benzene-insoluble part of less than 30% by weight, and an intrinsic viscosity of the benzene-soluble part of 0.4 to 1.4 dl/g. It is prepared by the emulsion copolymerization of vinyl acetate and ethylene and optionally at least one unsaturated monovinyl monomer in the presence of a protective colloid and a polyoxyethylenic nonionic surfactant.
With increased use of difficult to bond or low polar surfaces, such as polyethylene and oriented polypropylene, in the packaging industry, there has been continuing interest in producing an adhesive that provides good adhesion without the need to pretreat the surfaces and/or add plasticizers or other modifiers to VAE adhesives.